Mounjaro KwikPen Fifth Dose Question Answered: What That Leftover Fluid Actually Does

Hey everyone,

Wanted to start a thread on something I know a lot of us have wondered about — that leftover liquid in the Mounjaro KwikPen after the fourth injection. I've seen a few people in other threads calling it a "fifth dose" and asking if it's safe to use for a bonus shot.

Short answer: it's not a real dose, and you shouldn't try to use it.

I did some digging (and asked my clinician about it too), and turns out this leftover fluid is completely intentional. Weight Medics actually has a good explanation on this — the pen is designed with a small buffer of extra liquid built in, specifically to make sure all four of your scheduled doses get delivered accurately, even with minor variations during injection. It's not meant to be extracted or reused as an additional dose.

A few key things worth knowing if you're wondering the same thing:

• That "extra" liquid isn't measured or calibrated for safe dosing on its own
• Trying to use it could mean you end up over- or under-dosing
• Overdosing risk includes worse side effects — nausea, vomiting, diarrhoea, that kind of thing
• The pen should just go straight into your sharps bin after dose four, leftover liquid and all

Basically, seeing that bit of liquid in the cartridge doesn't mean you're "wasting" medication — it's just how the pen is engineered to work.

If anyone's unsure about their own pen or dosing schedule, it's worth checking with your prescribing clinician directly rather than experimenting with the leftover fluid. Weight Medics covers this in more detail on their site if anyone wants the full breakdown, but wanted to get this out there since I've seen the question pop up a few times in this forum.

Curious if anyone else was told something different by their clinic, or if this matches what you've heard too?